Neurostimulation systems, such as spinal cord stimulation (SCS) systems, deep brain stimulation systems and peripheral nerve stimulation systems, include at least one electrode positioned to enable stimulation of neural elements that are the target tissue (i.e. the tissue that, when sufficiently stimulated, will create the desired therapeutic effect). The electrodes are commonly mounted on a carrier and, in many instances, a plurality of electrodes are mounted on a single carrier. These carrier/electrode devices are sometimes referred to as “leads.” The electrodes may be used to cause nerves to fire action potentials (APs) that propagate along the neural fibers. More specifically, supplying stimulation energy to an electrode functioning as a cathode creates an electric potential field that causes depolarization of the neurons adjacent to the electrode. When the field is strong enough to depolarize (or “stimulate”) the neurons beyond a threshold level, the neurons will fire APs.
Stimulation energy may be delivered to the electrodes during and after the lead placement process in order to verify that the electrodes are stimulating the target neural elements and to formulate the most effective stimulation regimen. The regimen will dictate which of the electrodes are sourcing or returning current pulses at any given time, as well as the magnitude and duration of the current pulses. The stimulation regimen will typically be one that provides stimulation energy to all of the target tissue that must be stimulated in order to provide the therapeutic benefit (e.g., pain relief), yet minimizes the volume of non-target tissue that is stimulated. Thus, certain types of neurostimulation leads are typically implanted with the understanding that the stimulus pattern will require fewer than all of the electrodes on the leads to achieve the desired clinical effect; in the case of SCS, such a clinical effect is “paresthesia,” i.e., a tingling sensation that is effected by the electrical stimuli applied through the electrodes.
The present inventor has determined that conventional stimulus regimens, and the manner in which they are formulated, may be susceptible to improvement. For example, there are instances where the target tissue is not directly adjacent to an electrode and, because electrical field strength decreases exponentially with distance from the electrode, a relatively strong electric field must be created to generate APs in the target neural fibers. The electric field may, however, result in the generation of APs in the non-target fiber bundles between the electrode and the target fibers. The generation of APs in the non-target tissue may, in turn, lead to undesirable outcomes (e.g., discomfort) for the patient. The present inventor has determined that it may also be desirable in, for example, the context of leads that are oriented transverse to the target neural fibers, to selectively control the shape of the AP generating region in order to prevent the generation of APs in non-target fibers.